Abstract:

A method of reducing moisture in a fluorine-containing gas is described.
The method may include the steps of providing a purifier material that
includes elemental carbon, and flowing the unpurified fluorine-containing
gas having an unpurified moisture concentration over or through the
carbon-based purifier material. At least a portion of the moisture is
captured in the purifier material so that a purified fluorine-containing
gas that emerges downstream of the purifier material has a reduced
moisture concentration that is about 50% or less of the unpurified
moisture concentration.

Claims:

1. A method of reducing moisture in a fluorine-containing gas, the method
comprising:providing a purifier material comprising elemental
carbon;flowing an unpurified fluorine-containing gas having an unpurified
moisture concentration over or through the carbon-based purifier
material;capturing a portion of the moisture in the purifier material so
that a purified fluorine-containing gas that emerges downstream of the
purifier material has a reduced moisture concentration that is about 50%
or less of the unpurified moisture concentration.

13. A method for making a purified fluorine-containing gas, the method
comprising:contacting an unpurified fluorine-containing gas with a
carbon-based purifier material comprising elemental carbon;capturing a
portion of water vapor or a metal-containing impurity in the unpurified
gas such that the purified fluorine-containing gas has a water vapor or
metal-containing impurity concentration that is about 50% or less than
the unpurified gas.

25. A method for making a purified fluorine-containing gas, the method
comprising:contacting an unpurified fluorine-containing gas with a
carbon-based purifier material comprising elemental carbon;capturing an
impurity in the unpurified gas such that the purified fluorine-containing
gas has a concentration of the impurity that is about 50% or less than
the unpurified gas.

30. A system for in-situ generation of a purified fluorine-containing gas,
the system comprising:an unpurified fluorine-containing gas containing
moisture or a metal-containing impurity, wherein the unpurified
fluorine-containing gas is directed by an inlet conduit to a carbon-based
purifier material;the carbon-based purifier material comprising elemental
carbon and contained in a purification unit that is fluidly coupled to
the inlet conduit, wherein the carbon-based purifier material removes a
portion of the moisture or the metal-containing impurity from the
unpurified fluorine-containing gas; andan outlet port formed in the
purification unit through which the purified fluorine-containing gas
exits the purification unit, wherein the purified fluorine-containing gas
has a reduced moisture or metal-containing impurity concentration that is
about 50% or less of the unpurified moisture or metal-containing impurity
concentration.

31. The system of claim 30, wherein the purified fluorine-containing gas
has a reduced moisture or metal-containing impurity concentration that is
about 5% or less of the unpurified moisture concentration.

32. The system of claim 30, wherein the elemental carbon comprises beads
of microporous carbon having a size between about 0.0625 inches and about
0.5 inches.

34. The system of claim 30, wherein the purification unit comprises a
metal cylinder having an inlet port that can be reversibly coupled to the
inlet conduit, and positioned upstream of the outlet port also formed on
the cylinder, and wherein filter gaskets are placed over both the inlet
and outlet ports to prevent the purifier material from escaping the
cylinder.

35. The system of claim 30, wherein the outlet port is coupled to
semiconductor integrated circuit chip fabrication equipment.

Description:

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001]This application claims the benefit of U.S. Provisional Application
No. 61/039,725 by Millward et al, filed Mar. 26, 2008, and titled
"PURIFICATION OF FLUORINE CONTAINING GASES AND SYSTEMS AND MATERIALS
THEREOF." The entire contents of the U.S. Provisional patent application
is herein incorporated by reference for all purposes.

BACKGROUND

[0002]Fluorine-containing gases, including molecular fluorine, hydrogen
fluoride, nitrogen trifluoride, etc., find a variety of uses as etchants
and cleaning agents in semiconductor device fabrication processes. As
these processes have advanced, the purity requirements of the process
gases used has increased. For example, while older semiconductor
fabrication processes could tolerate levels of moisture impurities in
hydrogen fluoride in excess of 100 ppm, more recent processes require
that moisture level be reduced to 10 ppm or less.

[0003]Because fluorine containing process gases are normally highly
reactive, it is difficult to store them at high levels of purity for an
extended period. Thus, there is a need for purification methods and
systems that can purify the fluorine-containing gases as they are flowing
between a storage container and the point of use. These and other
problems are addressed by embodiments of the present invention.

BRIEF SUMMARY

[0004]Embodiments of the invention include methods of reducing moisture in
a fluorine-containing gas. The methods may include the steps of providing
a purifier material which includes elemental carbon, and flowing an
unpurified fluorine-containing gas having an unpurified moisture
concentration over or through the carbon-based purifier material. The
carbon-based purifier material captures a portion of the moisture in the
purifier material so that a purified fluorine-containing gas emerges
downstream of the purifier material with reduced moisture concentration
about 50% or less of the unpurified moisture concentration.

[0005]Embodiments of the invention also include methods or making a
purified fluorine-containing gas. The methods may include the steps of
contacting an unpurified hydrogen fluoride or molecular fluorine gas with
a carbon-based purifier material which includes elemental carbon, and
capturing a portion of water vapor or a metal-containing impurity in the
unpurified gas such that the purified hydrogen fluoride or molecular
fluorine gas has a water vapor or metal-containing impurity concentration
that is about 50% or less than the unpurified gas.

[0006]Embodiments of the invention still further include methods for
making a purified fluorine-containing gas. The methods may include the
steps of contacting an unpurified fluorine-containing gas with a
carbon-based purifier material comprising elemental carbon, and capturing
an impurity in the unpurified gas such that the purified
fluorine-containing gas has a concentration of the impurity that is about
50% or less than the unpurified gas.

[0007]Embodiments of the invention further include methods of reducing
moisture in a flow of hydrogen fluoride gas. The methods may include the
steps of heating a bed of microporous charcoal beads in a dry environment
to activate the bed, and flowing unpurified hydrogen fluoride gas, having
a unpurified moisture concentration, through the activated bed of
charcoal beads so that a portion of the moisture in the hydrogen fluoride
gas is captured by the bed. A purified hydrogen-fluoride gas emerging
from the bed has a reduced moisture concentration that is about 5% or
less of the unpurified moisture concentration.

[0008]Embodiments of the invention still further include systems for
in-situ generation of a purified fluorine-containing gas. The systems may
include an unpurified fluorine-containing gas containing moisture and/or
a metal-containing impurity, where the unpurified fluorine-containing gas
is directed by an inlet conduit to a carbon-based purifier material. The
systems may further include the carbon-based purifier material containing
elemental carbon, and contained in a purification unit that is fluidly
coupled to the inlet conduit. The carbon-based purifier material removes
at least a portion of the moisture and/or the metal-containing impurity
from the unpurified fluorine-containing gas. The systems may also include
an outlet port formed in the purification unit through which the purified
fluorine-containing gas exits the purification unit. The purified
fluorine-containing gas has a reduced moisture and/or metal-containing
impurity concentration that is about 50% or less of the unpurified
moisture concentration.

[0009]Additional embodiments and features are set forth in part in the
description that follows, and in part will become apparent to those
skilled in the art upon examination of the specification or may be
learned by the practice of the invention. The features and advantages of
the invention may be realized and attained by means of the
instrumentalities, combinations, and methods described in the
specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a flowchart showing selected steps in methods of reducing
one or more impurities in a fluorine-containing gas according to
embodiments of the invention;

[0011]FIG. 2 is a simplified schematic diagram showing selected components
in a system for the in-situ generation of a purified fluorine-containing
gas according to embodiments of the invention; and

[0013]Methods and systems are described for purifying fluorine-containing
gases with purifier materials that contain carbon. The carbon may be one
or more allotropes of elemental carbon, such as amorphous carbon in the
form of activated carbon, charcoal, activated charcoal, carbon black,
soot, and/or coal, among other allotropes of elemental carbon. The carbon
may also be an allotrope of elemental carbon having longer range order,
such as particles of graphite, graphene, carbon nanotubes, and/or
fullerenes, among other allotropes of elemental carbon.

[0014]The carbon-based purifier material may be used alone or in
combination with additional purifier materials to remove one or more
impurities from the fluorine-containing gases, including moisture and/or
other impurities. Traditionally, carbon-based purifier materials have not
been used to remove moisture from halogen gases, but these purifier
materials are effective to remove low concentrations of moisture (e.g.,
500 ppm or less) increasingly required for fluorine-containing gases used
in precision microelectronics fabrication applications. When extremely
low levels of moisture impurities in the fluorine-containing gas are
required, the carbon-based purifier materials may be prepared as
ultra-low emission (ULE) carbon, which is sufficiently dehydrated to
display hydrophilic properties, and can reduce moisture impurities in a
fluorine-containing gas without concurrently emitting water vapor back
into the purifying gas.

[0015]FIG. 1 shows a flowchart with selected steps in a method 100 of
reducing one or more impurities in a fluorine-containing gas according to
embodiments of the invention. The method 100 includes the step of
providing a purifier material that includes elemental carbon 102. As
noted above, the term elemental carbon refers to one or more allotropes
of solid phase carbon, such as activated carbon, charcoal, carbon black,
powdered graphite, etc. For example, the purifier material may be made
primarily or exclusively of a microporous carbon, such as microporous
charcoal. In addition to elemental carbon, carbon-based purifier material
may include organic-based carbon (e.g., substituted and unsubstituted
hydrocarbons), and organic polymers, including fluorinated polymers.

[0016]The carbon-containing purifier material may be made of substantially
homogeneous particles, or they may be dispersed as powders or coatings on
high surface area support materials. The particles can be shaped,
extruded, compacted, or formed into various forms, which may include
without being limited to pellets, spheres, irregular-shaped extrudates,
and variable-shaped extrudates, among other forms. The purifier materials
may also be layered particles comprising an outer layer of a first
material and a particle interior made of a second material, with one or
both layers containing carbon. Embodiments may also include a plurality
of layers surrounding the particle interior with one or more of the
layers containing carbon.

[0017]The elemental carbon may be provided in a conditioned state. This
may include dried carbon which has been exposed to a dry gas stream
(e.g., a flow of an inert gas such as helium, nitrogen, argon, or
mixtures of inert gases) under moderate to high temperatures (e.g.,
temperatures ranging from about 100° C. to about 1000° C.).
In some embodiments, the conditioned state may also include exposure of
the elemental carbon to a pure or diluted stream of molecular fluorine
gas (F2) in addition to (or in lieu of) other conditioning steps.

[0018]The term elemental carbon does not require the complete absence of
other elements. Carbon-containing purifier materials made from elemental
carbon may include minor amounts of additional elements, including
oxygen, nitrogen, hydrogen, and metals, that have not been completely
removed from the purifier. These additional elements may be unreacted
with the bulk elemental carbon, or may have reacted to form compounds in
the elemental carbon (e.g., oxidized carbon).

[0019]Conditioned carbon-containing purifier materials may include
ultra-low-emission (ULE) carbon materials which have been sufficiently
dehydrated to display hydrophilic properties that can further reduce
concentrations of moisture and other impurities without emitting water
vapor back into the purifying fluid (i.e., gas or liquid). Materials used
to make ULE carbon may include commercially available activated carbon,
such as high-hardness carbon beads sold by Advanced Technology Materials,
Inc., of Danbury, Conn. The stock material is conditioned in an ultra-dry
atmosphere at a sufficient temperature and time to reduce the level of
moisture in the material less than about 1 part-per-billion (ppb). The
dried product is referred to as ULE carbon due to its extremely low
emissions of moisture during purification. Additional details on the
types of materials and process conditions used to make ULE carbon can be
found in co-assigned U.S. Pat. No. 6,709,482, to Funke et al, issued Mar.
23, 2004, and titled "METHOD FOR REDUCING TRACE IMPURITIES FROM A
REACTIVE FLUID USING PRECONDITIONED ULTRA-LOW EMISSION CARBON MATERIAL"
the entire contents of which is herein incorporated by reference for all
purposes.

[0021]Method 100 may also include the step of flowing an unpurified
fluorine-containing gas over or through the carbon-based purifier
material 104. The unpurified fluorine-containing gas may include hydrogen
fluoride (HF) and/or molecular fluorine (F2) gas. Semiconductor
manufacturers are demanding increasingly high purity levels of HF and
F2 for semiconductor fabrication processes such as wafer etching and
chamber cleaning. Thus, method 100 may be used to produce high purity HF
and F2 (e.g., purity of 99.9%, 99.99%, 99.995%, etc.) at the time
and point-of-use by these processes.

[0022]The terms "purified" and "unpurified" fluorine-containing gas are
relative and may refer to a fluorine-containing gas that has been
previously purified, but can undergo additional purification using the
methods and systems described here. For example, a fluorine-containing
gas purified to 99.9% or higher purity may be considered an "unpurified"
gas when it contacts the above-described purifier materials which reduce
the concentration of one or more impurities in the gas even further.
Similarly, a "purified" fluorine-containing gas produced by the presently
described methods and systems may be considered an "unpurified" gas when
making contact with the same purifier material again, or making contact
with a different purifier material downstream from the first purifier
material.

[0024]The unpurified fluorine-containing gases include moisture (H2O)
as an impurity. They may also include additional impurities, including
metal containing contaminants such as volatile and non-volatile metal
fluorides, and halogen containing contaminants such as hydrogen chloride
(HCl), hydrogen bromide (HBr), and boron trichloride (BCl3), among
other types of fluorinated and chlorinated impurities. The additional
impurities may further include oxygen-containing compounds, such as
molecular oxygen (O2), carbon monoxide (CO), carbon dioxide
(CO2), among others. Impurities may also include fluorine-containing
gases when they are not the gas being purified. For example, hydrogen
fluoride (HF), sulfur hexafluoride (SF6), silicon tetrafluoride
(SiF4), nitryl fluoride (FNO2), sulfuryl difluoride
(SO2F2), carbonyl fluoride (COF2), and fluorinated
hydrocarbons such as tetrafluoromethane (CF4), hexafluoroethane
(C2F6), and trifluoromethane (CHF3), among other
fluorine-containing gases, may be impurities to be removed from other
fluorine-containing gases.

[0025]Method 100 may further include capturing a portion of the impurities
(e.g., moisture, metals, etc.) from the unpurified fluorine-containing
gas in the purifier material 106. When the impurities include moisture,
the amount of moisture captured may be such that the concentration of
moisture in the purified fluorine-containing gas is about 50 mol. % or
less than the unpurified moisture concentration. In further examples, the
moisture content may be reduced to about 15 mol. % or less, about 5 mol.
% or less, about 2 mol. % or less, about 1 mol. % or less, about 0.5 mol.
% or less, about 0.1 mol. % or less, about 0.01 mol % or less, etc., than
the unpurified moisture concentration. In terms of absolute concentration
of moisture in the fluorine-containing gas, the moisture levels may be
reduced from about 10,000 ppm-1 ppm in the unpurified gas to less than
about 1 ppm in the purified gas (e.g., about 50 ppb or less).

[0026]When the impurities include metal-containing compounds (e.g.,
halogenated metals), the amount of the metal-containing compounds
captured may be such that the concentration of metal concentration in the
purified fluorine-containing gas is about 50 mol. % or less than the
unpurified metal concentration. In further examples, the metal content
may be reduced to about 15 mol. % or less, about 5 mol. % or less, about
2 mol. % or less, about 1 mol. % or less, about 0.5 mol. % or less, about
0.1 mol. % or less, about 0.01 mol % or less, etc., than the unpurified
metal content. In terms of absolute concentration of metal-containing
impurities in the fluorine-containing gas, the metal impurities levels
may be reduced from about 10,000 ppm-1 ppm in the unpurified gas to less
than about 1 ppm in the purified gas (e.g., about 50 ppb or less).

[0027]Referring now to FIG. 2, a simplified schematic diagram showing
selected components in a system 200 for the in-situ generation of a
purified fluorine-containing gas according to embodiments of the
invention is shown. The system 200 may include a source of unpurified
fluorine-containing gas 202. This source 202 may include a storage vessel
(e.g., a high-pressure storage cylinder) holding the unpurified
fluorine-containing fluid (e.g., neat hydrogen fluoride). The unpurified
fluid from source 202 is directed to an inlet 204 of the purification
unit 206 by a leak tight fluid conduit 208. The rate of unpurified fluid
supplied to the purification unit may be controlled by a flow control
device 210, such as a pressure regulator, mass flow controller, etc.

[0028]The unpurified fluid entering inlet 204 makes contact with the
purifier material 212 held inside the purification unit 206. As noted
above, the purifier material 212 may include a carbon-based comprising
elemental carbon. It may also include one or more non-carbon-based
purifier materials that are mixed with the carbon-based purifier. For
example, the purifier material 212 may include a mixture of microporous
charcoal and one or more metal fluorides.

[0029]The purification unit 206 may also include an outlet port 214
through which the purified fluorine-containing gas exits the unit. The
purified fluorine-containing gas emerging from the purification unit 206
may have a reduced concentration of one or more impurities that is about
50 mol. % or less, 15 mol. % or less, 5 mol. % or less, etc., of the
unpurified impurity concentration. Specific examples of the impurities
removed from the purified fluorine-containing gas may include, moisture,
metal impurities such as volatile metal fluorines, and halogen impurities
such as hydrogen chloride (HCl), hydrogen bromide (HBr), boron
trichloride (BCl3), carbon monoxide (CO), carbon dioxide (CO2),
molecular oxygen (O2), and halogenated or non-halogenated
hydrocarbons, among other impurities.

[0030]System 200 may also include an application 216 to which the outlet
port 214 of the purification unit 206 is coupled. The application 216 may
be in close proximity (e.g., same room, same building, same site, etc.)
as system 200 so that the purified fluorine-containing gas is provided
in-situ to the application when needed. Examples of applications 216 may
include semiconductor fabrication systems among other applications.

[0031]In additional embodiments (not shown), a carbon-based purifier
material may be physically separated from other purifier materials in the
purification unit 206. For example, the purification unit 206 may be
partitioned into separate, fluidly communicating compartments, each of
which stores a different purifier material. The unpurified
fluorine-containing fluid may flow from inlet 204 into a first
compartment that contains a non-carbon based purifier material that
reduces the concentration of moisture (and possibly other impurities)
from in initial level (e.g., about 1 ppm to about 10,000 ppm) to a
reduced level (e.g., about 1 ppm or less). The partially purified gas
then flows to a second compartment in the purifier unit 206 that contains
a carbon-based purifier material that further reduces the moisture
concentration, and possibly other impurities as well. Such partitioned
purifier units may contain a plurality of compartments that hold purifier
material. They may be separated by mesh screens, check valves, etc., that
permit fluids to flow from one compartment to another while maintaining a
physical separation between the solid purifier materials.

Experimental

[0032]Purifier methods and systems are tested to measure their
effectiveness at removing impurities such as water vapor (i.e., moisture)
from fluorine-containing gases such as neat hydrogen fluoride. The
purifiers are assembled with various purifier materials and challenged
with HF containing ppm levels of water. The purifier materials tested may
include anhydrous aluminum fluoride, dehydrated aluminum fluoride
trihydrate, calcium fluoride, Drierite, elemental carbons, magnesium
fluoride, zirconium fluoride, potassium hexafluoronickelate, lithium
fluoride and nickel fluoride. In some examples, a carbon-based purifier
material is used in combination with other purifier materials that
contain little or no carbon. In still other comparative examples, the
non-carbon based purifier material is used exclusively to purify the
fluorine-containing gases.

[0033]The carbon-based purifier materials include commercially available
sources of microporous activated carbon (i.e., charcoal) with an average
size of, for example, about 0.0625 inches and surface area of about 1000
m2/g. In some instances, the carbon-based purifier materials are
conditioned before use in a purification system by, for example, being
exposed to a stream of a low-moisture, non-reactive gas such as dry
nitrogen (N2). Non-carbon based purifier materials may include
commercially available chips of anhydrous calcium sulfate (CaSO4)
with an average size of about 0.125 inches. They may also include porous
alumina beads, doped with oxides and hydroxides of cesium, with an
average size of about 0.125 inches, and extrudates of porous,
high-silicon zeolite with an average size of about 0.125 inches. Still
other examples of non-carbon based purifier materials include extrudates
of a fluoropolymer with sulfonic acid side groups that have an average
size of about 0.125 inches.

[0034]The non-carbon based purifier materials may also include inorganic
metal fluorides. The metal fluoride may start as powders that are pressed
into pellets, followed by crushing to obtain 1 to 2 mm-sized pieces or
smaller. The pressed pellets may include anhydrous AlF3,
K2NiF6, LiF, MgF2, CaF2, NiF2 and ZrF4,
which may be sieved to select sizes between 150 and 425 μm. Surface
areas for dehydrated AlF3.3H2O, anhydrous AlF3, MgF2,
CaF2 and LiF purifier media were found to be 77, 45, 42, 10, and 8
m2/g respectively.

[0035]In the case of the aluminum fluoride, attempts may be made to
fluorinate the aluminum oxide in an HF stream while purging away the
resulting water. The purpose is to retain the high surface area of the
material while rendering it chemically inert to HF. The resulting
purifier material has a surface area of 38 m2/g.

[0036]Purification units are assembled and loaded with the candidate
purifier materials. Purification containers are constructed of stainless
steel tubes fitted with isolation valves and 60 micron filter gaskets on
both ends. Most often the size is 1/2''×12'', but where more
material is available a larger 1'' or 2'' purifier may be used. Heat and
N2 purging are used in concert to dry and condition the purification
materials. Purification units are filled 3/4 full with purifier material
to allow space to preheat the purge N2. Heating profiles with
generally 1 LPM of purified N2 flow.

[0037]The performance of the purification units are evaluated in an FT-IR
system using an appropriate quantification method. The purification units
are installed in the testing system diagramed in FIG. 3 and atmospherics
are purged from the system using N2. Heat tracing maintained at
40° C. is applied to the purifier body and surrounding plumbing.
The HF cylinder produces variable water concentrations, presumably due to
temperature fluctuations. A 35° C. heating tape, and eventually a
heating blanket, may be applied to the HF cylinder.

[0038]A Nicolet 550 FTIR is equipped with a solid Ni 15 cm path-length
cell fitted with CaF2 windows for testing. Heat tracing on the cell
is maintained at 100° C. to prevent gas phase hydrogen bonding of
HF in the cell, and cell pressure is manually maintained at approximately
760 torr via inlet and outlet needle valves. A water quantification
method having an approximate MDL of 2 ppmv and a calibration range of
2-50 ppmv may be established in this testing system using the same
parameters. This method can be used throughout the experiments to
quantify the moisture in the gas stream.

[0039]Testing begins by checking the manifold and purifier respectively
for atmospherics using purified N2. Once the system is known to be
free of contaminants, HF is introduced into the FT-IR cell via the bypass
to establish the water concentration inherent in the HF challenge. Then
the HF is diverted through the purifier to measure its ability to remove
water.

[0040]Baseline water concentrations are established by sending the HF
challenge gas through the bypass to the FT-IR. The water baseline is
found to be variable both during and between tests, so the water content
in the challenge HF is periodically measured during the experiments. The
challenge HF is then passed through the purifier and FT-IR to measure the
water content in the purifier effluent. The testing includes the use of a
permeation tube and the Nicolet 550 FT-IR, D. A method may be developed
for water detection in HF (calibrated from 2 to 50 ppm).

[0041]Candidate purifier materials vary in their chemical compatibility
with HF, as well as their efficiency and capacity for removing water.
Some purifier materials had poor to fair compatibility with an HF gas
flow and generated some volatile compounds by chemical reaction when
exposed to HF, and/or were prone to liquefying. However, these materials
may have improved compatibility with other fluorine-containing gases.

[0042]Other purifier materials had good compatibility with impure HF gas
flows, but the efficiencies of some of these materials are difficult to
determine because the measured water concentration level rarely drops
below the moisture level in the challenge HF gas. This is the case for
NiF2, dehydrated AlF3.3H2O, and CaF2 samples.

[0043]After an initial water spike (common to all media tested), the water
concentrations from the LiF purifier drops to, and remains at, the level
of the challenge HF. This behavior indicates saturation of the LiF media.
The carbon-based purifiers have variable water concentrations in the
challenge HF, but the effluent seems to follow that concentration after
the initial spike.

[0044]Potassium hexafluoronickelate has the added benefit of changing
color from red to pale yellow when hydrated. Water removal efficiencies
are comparable to a larger AlF3 purifier. However, there are
uncertainties about longer term compatibility of this material with flows
of impure HF gas. A solid white crystalline material starts to deposit in
the IR cell after running the challenge HF gas through the
K2NiF6 purifier.

[0045]Zirconium fluoride is somewhat reactive with ambient humidity while
preparing the purifier material. After the oxide is removed by exposure
to HF, the purifier is reactivated. It achieves 80% efficiency after 2.2
hrs which is not a sufficient time to reach breakthrough. Based on the
75% efficiency of the first AlF3 purifier, a larger purifier of
smaller AlF3 particles is tested and found to have improved
efficiency.

[0046]A subset of purifier materials that includes ZrF4 and some
carbon-based materials are also tested for the removal of metal
impurities. The ZrF4 purifier effectively reduces the levels of Cr,
Cu and Zr in the gas flow. It appears to generate S, Ba and W, and to
leave the other metals at approximately the same levels inherent in the
control experiment. The carbon-based materials are also effective
removing an even larger group of metal impurities from the challenge HF
gas stream. They substantially removed Na, Mg, Al, P, Ca, Ti, Cr, Fe, Ni,
Cu, Zn, Sr, Zr and W, while reducing the concentrations of B and K.

[0047]Having described several embodiments, it will be recognized by those
of skill in the art that various modifications, alternative
constructions, and equivalents may be used without departing from the
spirit of the invention. Additionally, a number of well-known processes
and elements have not been described in order to avoid unnecessarily
obscuring the present invention. Accordingly, the above description
should not be taken as limiting the scope of the invention.

[0048]Where a range of values is provided, it is understood that each
intervening value, to the tenth of the unit of the lower limit unless the
context clearly dictates otherwise, between the upper and lower limits of
that range is also specifically disclosed. Each smaller range between any
stated value or intervening value in a stated range and any other stated
or intervening value in that stated range is encompassed. The upper and
lower limits of these smaller ranges may independently be included or
excluded in the range, and each range where either, neither or both
limits are included in the smaller ranges is also encompassed within the
invention, subject to any specifically excluded limit in the stated
range. Where the stated range includes one or both of the limits, ranges
excluding either or both of those included limits are also included.

[0049]As used herein and in the appended claims, the singular forms "a",
"an", and "the" include plural referents unless the context clearly
dictates otherwise. Thus, for example, reference to "a process" includes
a plurality of such processes and reference to "the purifier" includes
reference to one or more purifiers and equivalents thereof known to those
skilled in the art, and so forth.

[0050]Also, the words "comprise," "comprising," "include," "including,"
and "includes" when used in this specification and in the following
claims are intended to specify the presence of stated features, integers,
components, or steps, but they do not preclude the presence or addition
of one or more other features, integers, components, steps, acts, or
groups.